Low NOx stratified combustion of a compressed natural gas with an over-expansion injector

本研究旨在揭示利用超声速气流过膨胀特性的可行性。它会在燃料射流处产生比环境空气更低的压力区，环境空气有望被吸入燃料射流，燃料射流中心区域的富氧区会变得稀薄，从而导致燃烧温度降低，NOx生成减少。在对该特性在压力比和马赫数平面上的估计的基础上，对高速压缩机实现的高压、高温环境下的激波型进行了预测。当采用快速压缩机降低压缩比为5.8时，燃烧效率略有提高，CO含量略有减少，这表明过度膨胀条件的形成有影响。但检测到氮氧化物水平变化不大。因此，在较低的压缩比下实现了过膨胀效应，而超膨胀效应并不显著。为了在亚音速条件下将环境空气引入燃料射流，还测试了湍流混合夹带空气的利用，即所谓的本生灯机制。在常压条件下，通过稳态试验台实验，成功地观察到了空气引入现象。然而，在快速压缩机的实验中，这种空气引入效果甚微。由此也表明，引入空气对减少NOx排放的效果并不明显。在应用于发动机燃烧方面，研究了喷油器位置的影响，因为它通常局限于气缸盖。对喷油器定位、侧注和中心注进行了对比研究。中心喷射时，等量比高时燃烧效率提高，等量比低时燃烧效率下降。这是由于燃油射流与燃烧室壁面的相互作用使中心喷射的混合更好而产生的。从而明确了喷油器定位的重要性，并提出了未来喷油器参数确定策略的必要性。少

The present study aimed at revealing the feasibility of utilizing over-expansion behavior of supersonic gas flow. It would generate lower pressure area at the fuel jet than the ambient air, the ambient air was expected to be sucked into the fuel jet, and the richer zone in the central region of the fuel jet would become leaner resulting in lower combustion temperature and lower NOx formation. On the basis of the estimation of this behavior on the pressure ratio and the mach number plane, the shock wave pattern was predicted under the condition of high pressure and temperature ambient realized by a rapid compression machine. When the reduced compression ratio of 5.8 in the rapid compression machine was applied, slight increase of combustion efficiency and reduction of CO were observed, which suggested the effect of formation of over-expansion condition. However, little variation of NOx level was detected. Thus the over-expansion effect was realized at lower compression ratio and the ext … More ent is not so remarkable.For introducing ambient air into the fuel jet at subsonic condition, utilization of air entrainment with turbulent mixing, so-called Bunsen burner mechanism, was also tested. Under atmospheric condition with steady-state test rig experiment, the phenomenon of air introduction was successfully observed. However, in the experiment of rapid compression machine, little effect of such air introduction was obtained. Thus it was also shown that the air introduction is not so effective enough to reduce NOx emission.In terms of application to engine combustion, effect of injector positioning was examined since it is normally limited to cylinder head. Comparison study of injector positioning, side and central injection was carried out. With the central injection, combustion efficiency was increased at higher equivalence ratio condition but it deteriorated much at leaner condition. This would be generated by the better mixing of central injection due to the interaction between the fuel jet and the combustion chamber wall. Thus the importance of fuel injector positioning was clearly shown and the need of the future strategy of injection parameter determination was suggested. Less